This application relates to a self-regulated cooling system for switching power supplies using parasitic effects of switching.
A power supply is a device for the conversion of available power from one set of characteristics to another set of characteristics to meet specified requirements. Power supplies include ac-ac converters (eg. frequency changers, cycloconverters), ac-dc converters (eg. rectifiers, offline converters), dc-ac converters (also called inverters), and dc-dc converters (also simply called xe2x80x9cconvertersxe2x80x9d).
A xe2x80x9cswitching power supplyxe2x80x9d or xe2x80x9cswitching-mode power supplyxe2x80x9d is a power supply that provides the conversion function through low loss components such as capacitors, inductors, and transformers, and the use of switches (eg. transistors) that are in one of two states, namely on or off. The advantage is that the switch dissipates very little power in either of these two states and power conversion can be accomplished with minimal power loss, which equates to high efficiency.
However, in designing and building electrical circuits for such power supplies, it is often necessary to include xe2x80x9csnubberxe2x80x9d circuits to dampen spurious transients or oscillations. Also, when the switching device is turned off, overvoltages appear due to the parasitic inductances in the circuit. These overvoltages increase as the current to be switched increases because the amount of energy stored in the leakage inductance is proportional to the square of the current that goes through it. It is practically impossible to build a transformer with no leakage inductance.
Snubber circuits are well known in the prior art. A standard snubber circuit is described in U.S. Pat. No. 3,098,949 issued Jul. 23, 1963 to Goldberg.
A simple design for a snubber is simply a capacitor that shunts the switching transistor. A capacitor in parallel with the switch reduces the rate of rise of voltage across the switch when the switch is off, absorbing energy that would have had to be dissipated by the switch. In other words, the capacitor acts as an initial short in passing the current of the voltage transient around the switch until the capacitor charges up. However, such a design has the problem that the snubber capacitor charges up with the current in the transient spike. This charge must be bled off the snubber capacitor before the next cycle of the switch or the snubber capacitor will not be effective to snub the next transient. This bleeding off of charge occurs through the switch the next time the switch is turned on. This increases the electrical stress on the switch.
Typically, a lone capacitor is not used for the snubber without an accompanying resistor. A resistor-capacitor (xe2x80x9cRCxe2x80x9d) snubber circuit employs a resistor in series with a capacitor across the switch. The RC snubber provides both turn-off snubbing and damping of voltage oscillations across the switch, but it incurs a relatively high power loss. The resistor dissipates, as heat, some of the charge from the capacitor to prepare for the next switch turn-off. Further, the capacitor and the parasitic inductance together act as a tuned circuit and can oscillate or xe2x80x9cringxe2x80x9d, and so the addition of a series resistance sufficient to critically damp the circuit is also necessary to suppress this ringing. However, this additional series resistance also slows down the process of charging and discharging the snubber capacitor. The slower charging tends to diminish somewhat the effectiveness of the snubber.
To regain the snubbing effectiveness, a diode may be placed in parallel with the resistor such that the voltage transient will be of the proper polarity to forward bias the diode and charge the snubber capacitor through the diode. Then, to protect the diode from being destroyed by excessive current, a small current-limiting resistor is placed in series with the diode. However, the diode is reverse biased during the discharge cycle to ready the snubber capacitor for the next transient, so discharging must occur through the resistor that is in parallel with the diode. This slows the discharge rate and places a limitation on the maximum rate at which the switch may be operated since the next cycle cannot start until the snubber capacitor is fully discharged. Also, the resistor dissipates the charge in the form of heat which can damage the semiconductors, and therefore may give rise to an increased need for forced cooling.
An alternative approach is to place a diode in series with a resistor and capacitor in a resistor-capacitor-diode (xe2x80x9cRCDxe2x80x9d) snubber circuit. The diode, when forwardly biased, provides a mechanism to charge the snubber capacitor therethrough in preparation for the switch to turn on. With respect to either the RC snubber circuit or the RCD snubber circuit, the following principles apply: first, the capacitor is usually larger than the junction capacitance of the semiconductor switch so that the rising of the switch voltage is relatively slow, thereby reducing the voltage overshoot; second, the resistor provides damping to reduce voltage oscillations across the switch, but the resistor also dissipates energy in the form of heat. Finally, selection of the capacitor and resistor includes tradeoffs. More specifically, a larger capacitor reduces the transients, but increases the power dissipation associated therewith. When power levels are high enough, forced cooling is required to reduce the size of heat sinks to protect the semiconductors, depending on the heat sink temperature. In other words, an optimal design is difficult to achieve for any snubber circuit which utilizes a resistor to dissipate energy.
Even though most snubber circuits use resistors to dissipate extraneous energy as heat, attempts have been made to recover the wasted energy and put it to a useful application rather than dissipate it as heat. Some snubber circuits recycle the otherwise wasted energy back to the input terminal or to external loads. U.S. Pat. No. 4,438,486 issued Mar. 20, 1984 to Ferraro describes a snubber circuit comprising a diode and capacitor in series, together with an LED which, through a phototransistor, controls an oscillator and field effect transistor so as to cause a transformer to generate a current through a diode to charge a battery. However, the snubber circuit taught by Ferraro is complex and does not apply the parasitic inductive energy in a manner useful to the protection of the switches themselves.
Accordingly, what is needed in the art is a snubber circuit for semiconductor switches that minimizes overvoltages to thereby reduce the power losses associated with the switches and oscillations in both voltage and current therefrom and reduces the thermal effects of the snubber circuit on the switches employing the snubber circuit to advantage.
In accordance with the invention, a system and method for cooling switching power supplies involve a circuit which collects from a semiconductor switch parasitic inductive energy which would normally be dissipated as heat in a typical RC snubber circuit, and uses that waste energy to power a cooling element such as a fan, a water pump, a Peltier device, or the like that protects the semiconductor switch. Furthermore, the circuit provides power to the cooling element in proportion to the amount of current delivered to the switch, and so the circuit automatically adjusts itself to the level needed to protect the switch. This results in a self-regulated cooling system for protecting switching power supplies, making useful application of parasitic effects of switching that would otherwise be dissipated as potentially harmful heat.